Engines may be operated using boosting devices, such as turbochargers or superchargers, to increase mass airflow into a combustion chamber. Turbochargers and superchargers compress intake air entering the engine using an intake compressor. Further, one or more intake charging devices may be staged in series or parallel to improve engine boost response.
One example of a multi-staged boosted engine is shown by Kawamura et al. in U.S. Pat. No. 6,938,420. Therein, an electric supercharger driven by an electric motor and an electric supercharger bypass valve (ESBPV) are staged downstream of a turbocharger. During conditions when the turbocharger compressor is not spun up, the ESBPV may be closed and the electric supercharger may be rotated to provide a transient positive boost pressure in order to reduce turbo lag. Then, when the turbocharger compressor is sufficiently spun up, the ESBPV may be opened and the electric supercharger may be disabled, allowing the turbocharger to provide the desired boost pressure.
However, the inventors herein have recognized potential issues with such systems. As one example, if the electric supercharger is operated aggressively to reduce turbo lag, electric boost overshoot may occur, which may be difficult to control. In particular, due to hardware constraints, it may not be possible to brake the electric motor and provide negative torque to slow down the electric supercharger shaft speed responsive to an electric boost overshoot. Instead, the motor may be disabled enabling the high electric supercharger speeds to be reduced using natural decay including resistive effects of friction and air resistance. However, in the meantime, the actual boost pressure may continue to overshoot resulting in excessive engine torque output. During this time, the engine intake throttle may not have the bandwidth to react to the fast pressure build-up. Consequently, any throttle adjustments may result in actual manifold pressure overshooting the desired manifold pressure, further contributing to the boost overshoot. To leverage the natural decay of the supercharger speed to address the boost overshoot, the electric supercharger disabling may need to be commanded before the desired boost pressure or target torque is reached. However this increases the time to torque and results in a boost lag even with the electric supercharger operating. As such, the excess boost and excess torque can result in drivability issues.
In one example, the issues described above may be addressed by a method for a boosted engine, comprising: while a downstream compressor spins up, accelerating an upstream compressor with a bypass valve coupled in a bypass across the first compressor closed to provide a flow of compressed air to a piston engine and, in response to a boost pressure overshoot, opening the bypass valve. In this way, boost pressure overshoot may be more accurately controlled while expediting a time to torque.
As one example, an electric supercharger (ES) including a compressor driven by an electric motor may be staged upstream of a turbocharger (TC) including a compressor driven by an exhaust turbine. An electric supercharger bypass valve (ESBPV) may be coupled in a bypass around the ES. To reduce turbo lag, while the turbocharger compressor spins up, the ESBPV may be closed while the electric supercharger is transiently operated via the electric motor to provide positive pressure. In response to a boost pressure overshoot experienced downstream of the ES compressor while the TC compressor is still spinning up (that is, an electric boost overshoot condition), the ESBPV may be transiently opened to rapidly bleed down the electric boost pressure provided by the electric supercharger. In addition, the electric supercharger may be concurrently disabled and decelerated. Further, the supercharger adjustments may be provided in the complementary frequency band as adjustments to a wastegate coupled to the exhaust turbine of the TC, allowing for a faster and more accurate regulation of the TC compressor speed. In particular, the wastegate control loop may be tuned more aggressively since the fast dynamics of the ESBPV would be able to damp any oscillations and reduce the boost pressure overshoot.
In this way, airflow through an electric supercharger compressor can be substantially immediately limited, allowing for expedited electric boost overshoot control. The technical effect of opening a bypass valve around the electric supercharger while a downstream turbocharger spins up is that boosted airflow to the engine can be more rapidly reduced to a level desired based on driver demand. This enables boosted air pressure to be regulated more quickly, and excess engine torque delivery to be reduced. By concurrently decelerating the supercharger, the boost pressure may not be affected by delays incurred in supercharger speed decay. In addition, if there is a sudden operator change of mind while the turbocharger is spinning up, the ESBPV can be closed and the ES accelerated to rapidly meet the driver demand without degrading the time to torque. Further, the ES and ESBPV adjustments may be provided in coordination with throttle adjustments and in the complementary frequency band as wastegate adjustments, allowing for a faster and more accurate regulation of the boost pressure. Further, the wastegate control loop may be tuned more aggressively.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.